Generally, a motion simulator refers to a device which simulates motions of objects in a wide space such as an airplane or an automobile and allows people to feel similar moving sensations within a limited space.
As a general motion simulator such as the above, a 6 DOF (degree of freedom) motion simulator 100 in which a movable frame 120 is driven by six actuators 131, 132, 133, 134, 135, 136 is depicted in FIGS. 1 to 3b. 
As depicted in FIG. 1, the conventional 6 DOF motion simulator 100 has a structure which includes a stationary frame 110, a movable frame 120 and a plurality of actuators 131, 132, 133, 134, 135, 136.
The stationary frame 110 is installed fixedly against the ground (gravity field). The movable frame 120 is disposed above the gravitational direction of the stationary frame 110. An operating chamber 140 is disposed on the top surface of the movable frame 120.
The plurality of actuators 131, 132, 133, 134, 135, 136 are disposed between the stationary frame 110 and the movable frame 120. Electric, hydraulic, or pneumatic actuators are generally used for each of the actuators 131, 132, 133, 134, 135, 136.
Each of the actuators 133, 132, 133, 134, 135, 136 is rotatably connected at both ends thereof to the stationary frame 110 and the movable frame 120 by respective pairs of universal joints 131a and 131b, 132a and 132b, 133a and 133b, 134a and 134b, 135a and 135b, 136a and 136b. 
The conventional 6 DOF motion simulator 100 configured as the above allows the passenger 170 in the operating chamber 140 to feel moving sensations similar to those felt when actually riding an airplane or automobile by driving the plurality of actuators 131, 132, 133, 134, 135, 136 and thereby moving the movable frame 120.
For instance, for a racing car that has suddenly taken off and continues to accelerate, the passenger feels sensations of being pulled backward due to acceleration, and this sensation is continued while acceleration after start is being progressed.
To create such sensation, the motion simulator 100 drives the plurality of actuators 131, 132, 133, 134, 135, 136 and firstly accelerates the movable frame 120 forward, as depicted in FIG. 2a. In the above case, the passenger 170 within the operating chamber 140 feels a pulling sensation from the rear due to the force of inertia.
However, because the range of motion of the motion simulator 100 has a limit, the movable frame 120 which has been accelerated and moved forward shortly falls within this limit. At this time, as depicted in FIG. 2b, when the movable frame 120 is rotated clockwise, the passenger 170 continues to feel the sensation due to gravity.
On the other hand, as another example, for an automobile turning along a large curve, the passenger feels a pushing sensation to the outer direction of the curve due to centrifugal force, and continues to feel this sensation while the turning is being progressed.
To create such sensation, the motion simulator 100 actuates the plurality of actuators 131, 132, 133, 134, 135, 136 and firstly accelerates the movable frame 120 to the side direction, as depicted in FIG. 3a. In the above case, the passenger 170 within the operating chamber 140 feels a sensation of being pushed in the opposite direction of the movement due to the force of inertia.
However, also for this case, because the range of motion of the motion simulator 100 has a limit, the movable frame 120 which has been accelerated and moved to the side direction shortly falls within this limit. At this time, as depicted in FIG. 3b, when the movable frame 120 is rotated clockwise, the passenger 170 continues to feel said sensation.
On the other hand, in FIGS. 4 to 6, as another example of the conventional motion simulator, a 3 DOF motion simulator 101 of which the movable frame 120 is driven by three actuators 131′, 132′, 133′ is depicted.
According to FIGS. 4 to 6, the configuration of the conventional 3 DOF motion simulator 101 is identical to that of the 6 DOF simulator except that the former has three actuators 131′, 132′, 133′ and that it is provided with a separate support member 150 to limit the occurrence of unintended forward/backward linear motion, left/right linear motion, and rotating motion centered on the top, bottom axes perpendicular to the surface of the movable frame 120.
Therefore, in describing the configuration of the 3 DOF motion simulator 101, same reference numbers are designated for parts identical to those of the 6 DOF motion simulator, and the descriptions thereof are omitted.
Meanwhile, as mentioned above, because all motions of the movable frame 120 can not be restrained with only the actuators 131′, 132′, 133′, in the depicted conventional 3 DOF motion simulator 101, there is provided a separate support member 150 for limiting the occurrence of unintended motion to the movable frame 120.
The support member 150 is composed of a cylinder 151 which is fixed on the stationary frame 110, a piston 152 which moves up and down along the cylinder 151, and a universal joint 153 which connects the piston 152 and the movable frame 120.
In the case of the conventional 3 DOF motion simulator 101 configured as the above, because there is no DOF to the horizontal direction, that is, the direction perpendicular to gravity, when creating continuous accelerating motion or rotating motion as mentioned above, only the force of gravity is used.
Namely, to create a linear accelerating sensation, the motion simulator 101 drives the plurality of actuators 131′, 132′, 133′ and rotate the movable frame 120 and thereby allows the passenger 170 to feel a rearward pulling sensation, as depicted in FIG. 5.
In addition, to create rotating movement, the motion simulator 101 drives the plurality of actuators 131′, 132′, 133′ and rotate the movable frame 120 and thereby allows the passenger 170 to feel a pushing sensation to the other side, as depicted in FIG. 6.
However, according to the conventional motion simulators 100, 101 configured as the above, both simulators have a structure in which the center of gravity of the passenger 170 is above the center of rotation of the movable frame 120.
Due to the above, when representing acceleration from continuous linear acceleration or from centrifugal motion to the side direction, that is, when the movable frame 120 is tilted to utilize gravity, there is the problem of occurrence of undesired acceleration.
This awkward sensation (that is, force) may be expressed with the following equation.Ap=Av+A×Rpv+ω×ω×Rpv                wherein, Ap is the acceleration vector felt by the passenger of the motion simulator, Av is the acceleration vector of the moving movable frame of the motion simulator, A is rotational acceleration vector of the movable frame, Rpv is the relative position vector of the passenger on top of the motion plate, and ω is the rotational velocity vector.        
The awkward sensation is sum of the calculation value of the cross product of A and Rpv vectors, which is A×Rpv, and the calculation value of the cross product of ω, ω, Rpv vectors, which is ω×ω×Rpv. Among these, the sensation expressed by A×Rpv gives the most uncomfortable feeling from forward/backward and lateral movement. The present invention eliminates an acceleration factor that is exerted oppositely to the acceleration sensation expected by the passenger.
That is, in the structure of conventional motion simulators 100, 101, because the center of gravity of the passenger 170 exists vertically above the center of rotation of the movable frame 120, when starting to rotate the movable plate, the value of the A×Rpv vector becomes the opposite direction of the acceleration intended to be created.
A graph displaying the above is shown in FIG. 7. The dotted line in FIG. 7 represents the acceleration felt by the driver of an automobile which is suddenly stopped or driven on a curve, and the solid line represents the acceleration/deceleration sensed by the passenger riding on the motion simulator driven by inputting the signals.
In FIG. 7, as shown by the pointed portions bulging out to the minus region in the opposite direction of the changes in the reference signals, in contrary to the intended pushing to one side sensation, a sudden attraction to the opposite side is experienced at the time of sudden change to the acceleration.
As a result of such problems, as shown by the solid line of FIG. 7, a moving sensation in the opposite direction of the moving sensation intended to be created (dotted line of FIG. 7) is applied, and furthermore, the time taken to track the intended moving sensation is delayed. This means a decline in actuality experienced by the passenger.
In the foregoing, the problems of the conventional motion simulator has been described taking the 6 DOF and 3 DOF motion simulators as two types of examples. However, although the extent may vary, the above mentioned problems of conventional motion simulators occur in all motion simulators having different degrees of freedom.